<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>test</title>
<script type="text/javascript" language="javascript">
function showadv() {

if(document.getElementById("form1").gaoji.checked == true) {
document.getElementById("level").style.display = "";

} else {
//document.forms["form1"];
document.getElementById("level").style.display = "none";

}

}
</script>
</head>

<body>
<form id="form1" name="form1"  runat="server">
<input type="checkbox" id="gaoji" onclick="showadv()" />
<div id="level" style="display:none">
test
</div>
</form>
</body>
</html>